Buggy code:
@ Override public void onClick ( View view ) { Button button = ( ( Button ) ( view ) ) ; switch ( button . getId ( ) ) { case id . button0 : handleNumbers ( "0" ) ; break ; case id . button1 : handleNumbers ( "1" ) ; break ; case id . button2 : handleNumbers ( "2" ) ; break ; case id . button3 : handleNumbers ( "3" ) ; break ; case id . button4 : handleNumbers ( "4" ) ; break ; case id . button5 : handleNumbers ( "5" ) ; break ; case id . button6 : handleNumbers ( "6" ) ; break ; case id . button7 : handleNumbers ( "7" ) ; break ; case id . button8 : handleNumbers ( "8" ) ; break ; case id . button9 : handleNumbers ( "9" ) ; break ; case id . buttonAdd : handleOperators ( "+" ) ; break ; case id . buttonMinus : handleOperators ( "-" ) ; break ; case id . buttonMultiple : handleOperators ( "*" ) ; break ; case id . buttonDivided : handleOperators ( "//" ) ; break ; case id . buttonDot : handleDot ( ) ; break ; case id . buttonNega : handleNegative ( ) ; break ; case id . buttonParenthesis : handleParenthesis ( ) ; break ; case id . buttonPercentage : if ( isEvalon ) { handleEval ( ) ; } else { handlePercentage ( ) ; } break ; case id . buttonFactorial : handleFactorial ( ) ; break ; case id . buttonSin : if ( is2ndOn ) { if ( isRadian ) { handleFunction ( "asinrad(" , "asin(" , true ) ; } else { handleFunction ( "asin(" , "asin(" , true ) ; } } else { if ( isRadian ) { handleFunction ( "sinrad(" , "sin(" , true ) ; } else { handleFunction ( "sin(" , "sin(" , true ) ; } } break ; case id . buttonCos : if ( is2ndOn ) { if ( isRadian ) { handleFunction ( "acosrad(" , "acos(" , true ) ; } else { handleFunction ( "acos(" , "acos(" , true ) ; } } else { if ( isRadian ) { handleFunction ( "cosrad(" , "cos(" , true ) ; } else { handleFunction ( "cos(" , "cos(" , true ) ; } } break ; case id . buttonTan : if ( is2ndOn ) { if ( isRadian ) { handleFunction ( "atanrad(" , "atan(" , true ) ; } else { handleFunction ( "atan(" , "atan(" , true ) ; } } else { if ( isRadian ) { handleFunction ( "tanrad(" , "tan(" , true ) ; } else { handleFunction ( "tan(" , "tan(" , true ) ; } } break ; case id . buttonLn : handleFunction ( "log(" , "ln(" , true ) ; break ; case id . buttonLog : handleFunction ( "log10(" , "Log(" , true ) ; break ; case id . buttonPi : handleFunction ( "PI" , "π" , false ) ; break ; case id . buttonE : handleFunction ( "e" , "e" , false ) ; break ; case id . button2nd : if ( is2ndOn ) { is2ndOn = false ; sin . setText ( "sin" ) ; cos . setText ( "cos" ) ; tan . setText ( "tan" ) ; sin . setBackgroundResource ( button_lightgray ) ; cos . setBackgroundResource ( button_lightgray ) ; tan . setBackgroundResource ( button_lightgray ) ; sin . setTextColor ( Color . parseColor ( "#000000" ) ) ; cos . setTextColor ( Color . parseColor ( "#000000" ) ) ; tan . setTextColor ( Color . parseColor ( "#000000" ) ) ; <START_BUG> buttonAns . setText ( "√" ) ; <END_BUG> buttonAns . setBackgroundResource ( button_lightgray ) ; buttonAns . setTextColor ( Color . parseColor ( "#000000" ) ) ; } else { is2ndOn = true ; sin . setText ( "asin" ) ; cos . setText ( "acos" ) ; tan . setText ( "atan" ) ; sin . setBackgroundResource ( button_green2 ) ; cos . setBackgroundResource ( button_green2 ) ; tan . setBackgroundResource ( button_green2 ) ; sin . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; cos . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; tan . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; buttonAns . setText ( "x" ) ; buttonAns . setBackgroundResource ( button_green2 ) ; buttonAns . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; } break ; case id . buttonDegree : isRadian = false ; deg . setBackgroundResource ( button_green2 ) ; deg . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; rad . setBackgroundResource ( button_gray ) ; rad . setTextColor ( Color . parseColor ( "#D7D7D7" ) ) ; break ; case id . buttonRadian : isRadian = true ; rad . setBackgroundResource ( button_green2 ) ; rad . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; deg . setBackgroundResource ( button_gray ) ; deg . setTextColor ( Color . parseColor ( "#D7D7D7" ) ) ; break ; case id . buttonAns : if ( is2ndOn ) { handleFunction ( "x" , "x" , false ) ; buttonPercentage . setBackgroundResource ( button_red ) ; buttonPercentage . setTextColor ( Color . parseColor ( "#FFFFFF" ) ) ; buttonPercentage . setText ( "EVAL" ) ; isEvalon = true ; equal . setBackgroundResource ( button_red ) ; equal . setText ( "DRAW" ) ; } else { handleAns ( ) ; } break ; case id . buttonDEL : handleDel ( ) ; break ; case id . buttonAC : handleAC ( ) ; break ; case id . buttonRoot : handleFunction ( "sqrt(" , "√(" , true ) ; break ;

Fix:
buttonAns . setText ( "Ans" ) ;